The considered structures consist of an armchair GNR coupled with
two electrodes which may be described either by graphitic (Fig.
1(a)) or normal-conducting (Fig. 1(b)) leads. In the simplest
consideration, the normal-conducting leads are modeled by square
lattices [1]}, [2]}, [3]}. A ferromagnetic gate is
assumed to create a potential barrier which controls the Fermi level
locally and to induce an exchange splitting into the device. To
model the structures, we use the single band tight binding
Hamiltonian
\(\hat{H} = \hat{H}_L + \hat{H}_D + \hat{H}_R + \hat{H}_C\) 
